Earth
Science, 10th edition
Chapter
16: Moisture, Clouds, and Precipitation
I. Changes of state of water
A. Heat energy
1.
Measured in calories – one calorie is the heat
necessary to raise the temperature
of one gram of
water one degree Celsius
2. Latent heat
a. Stored or hidden heat
b. Not derived from
temperature change
c. Important in atmospheric
processes
B. Three states of matter
1. Solid
2. Liquid
3. Gas
C. To change state, heat must be
1. Absorbed, or
2. Released
D. Processes
1. Evaporation
a. Liquid is changed to gas
b.
600 calories per gram of water are added – called latent heat of vaporization
2. Condensation
a. Water vapor (gas) is
changed to a liquid
b. Heat energy is released –
called latent heat of condensation
3. Melting
a. Solid is changed to a
liquid
b. 80
calories per gram of water are added – called latent heat of melting
4. Freezing
a. Liquid is changed to a
solid
b. Heat is released – called
latent heat of fusion
5. Sublimation
a. Solid is changed directly
to a gas (e.g., ice cubes shrinking in a freezer)
b.
680 calories per gram of water are added
6. Deposition
a. Water vapor (gas) changed
to a solid (e.g., frost in a freezer compartment)
b. Heat is released
II. Humidity
A. Amount of water vapor in the air
1. Saturated air is air that is
filled with water vapor to capacity
2. Capacity is temperature
dependent – warm air has a much greater capacity
3. Water vapor adds pressure
(called vapor pressure) to the air
B. Measuring humidity
1. Relative humidity
a. Ratio of the air's actual water vapor
content compared with the amount of
water vapor required for saturation at that temperature (and
pressure).
b. Expressed as a percent
c. Saturated air
1. Content equals capacity
2. Has a 100% relative
humidity
d. Relative humidity can be
changed in two ways
1. Add or subtract
moisture to the air
a. Adding moisture
raises the relative humidity
b. Removing moisture
lowers the relative humidity
2. Changing the air
temperature
a.
Lowering the temperature raises the relative
humidity
b. Raising the temperature lowers the relative
humidity
e. Dew point temperature
1. Temperature to which a
parcel of air would need to be cooled to reach saturation
2. Cooling the air below
the dew point causes condensation
a. e.g., dew, fog, or
cloud formation
b. Water vapor
requires a surface to condense on
III. Adiabatic heating/cooling
A. Adiabatic temperature changes occur
when
1. Air is compressed
a. Motion of air molecules
increases
b. Air will warm
c. Descending air is
compressed due to increasing air pressure
2. Air expands
a. Air parcel does work on the
surrounding air
b. Air will cool
c. Rising air will expand due
to decreasing air pressure
B. Adiabatic rates
1. Dry adiabatic rate
a. Unsaturated air
b. Rising air expands and
cools at 1°C per 100 meters (5.5 °F per 1000 feet)
c. Descending air is
compressed and warms at 1°C per
100 meters
2. Wet adiabatic rate
a. Commences at condensation
level
b. Air has reached the dew
point
c. Condensation is occurring
and latent heat is being liberated
d. Heat released by the condensing
water reduces the rate of cooling
e. Rate varies from 0.5°C to 0.9°C per 100 meters
IV. Processes that lift air
A. Orographic
lifting
1. Elevated terrains act as
barriers
2. Result can be a rain shadow
desert
B. Frontal wedging
1. Cool air acts as a barrier to
warm air
2. Fronts are part of the storm
systems called middle-latitude cyclones
C. Convergence where the air is flowing
together and rising
D. Localized convective lifting where
unequal surface heating causes localized pockets of
air to rise because of their buoyancy
V. Stability of air
A. Types of stability
1. Stable air
a. Resists vertical
displacement
1. Cooler than surrounding
air
2. Denser than surrounding
air
3. Wants to sink
VI. Condensation and cloud formation
A. Condensation
1. Water vapor in the air changes
to a liquid and forms dew, fog, or clouds
2. Water vapor requires a surface
to condense on
a. Possible condensation
surfaces on the ground can be the grass, a car window, etc.
b. Possible condensation
surfaces in the atmosphere are tiny bits of particulate matter
1. Called condensation
nuclei
2. Dust, smoke, etc.
3. Ocean salt crystals
which serve as hygroscopic ("water seeking") nuclei
B. Clouds
1. Made of millions and millions of
a. Minute water droplets, or
b. Tiny crystals of ice
2. Classification based on
a. Form (three basic forms)
1. Cirrus – high, white,
thin
2. Cumulus
a. Globular cloud
masses
b.
Often associated with fair weather
3. Stratus
a. Sheets or layers
b. Cover much or all
of the sky
b. Height
1. High clouds
a.
Above 6000 meters
b. Types
1. Cirrus
2. Cirrostratus
3. Cirrocumulus
2. Middle clouds
a.
2000 to 6000 meters
b. Types (alto as
part of the name)
1. Altocumulus
2. Altostratus
3. Low clouds
a.
Below 2000 meters
b. Types
1. Stratus
2. Stratocumulus
3. Nimbostratus
(nimbus means "rainy")
4. Clouds of vertical
development
a.
From low to high altitudes
b. Called
cumulonimbus
c. Often produce
1. Rain showers
2. Thunderstorms
VII. Fog
A. Considered an atmospheric hazard
B. Cloud with its base at or near the
ground
C. Most fogs form because of
1. Radiation cooling, or
2. Movement of air over a cold
surface